Stirling engine

ABSTRACT

A closed fluid working system for a Stirling engine is disclosed. The working system has a plurality of chambers subdivided by double-acting pistons operating therein; the subdivided chambers are respectively hot and cold and connected in series whereby a hot chamber is always in communication with a cold chamber of the next most adjacent cylinder. Parallel arranged gas flow paths are interposed in each communication between hot and cold chambers, and regenerator-cooler mechanisms are disposed in each of said parallel arranged paths. Control means are employed to permit flow through one or more of said parallel paths or flow of different levels through all of the paths, during different load conditions of the engine to permit the regenerator-cooling capacity to be tuned to the needs of the engine.

BACKGROUND OF THE INVENTION

The Stirling cycle requires two volumes interconnected by a dead spacehaving a regenerator included in the latter. One of the volumes is anexpansion space, maintained at a generally high temperature, and theother volume is a compression space maintained at a relatively lowtemperature. The regenerator is equivalent to a thermodynamic sponge,alternately releasing and absorbing heat when gases are transferredbetween the two volumes. The dead space consists of that part of theworking space not swept by any of the pistons operating within theexpansion and compression volumes; this dead space typically willinclude cylinder clearance spaces, void volumes of the regenerator orheat exchangers, and the internal volume of the associated ducts andports interconnecting the two volumes.

The amount of flow through the dead space, during each cycle ofoperation of the Stirling engine is important because flow lossestherein affect the net cycle output and the efficiency of the engine.Emperical data has been employed to date to guide the design of the deadspace and thereby the regenerator configuration of a Stirling engine.

For example, it has been found that the desirable characteristics for aregenerator matrix should comprise: (a) for maximum heat capacity, alarge solid matrix; (b) for maximum heat transfer, a large,finely-divided matrix; (c) for minimum flow losses, a small, highlyporous matrix; and (d) for minimum dead space, a small, dense matrix.Clearly, it is impossible to satisfy all of these conflictingrequirements. Therefore, with the present state of art for the Stirlingcycle, a compromise has been employed; this compromise has resulted inwhat is known as a fixed regenerator design which will not vary involume or flow capacity in spite of the fact that the engine itselfprovides different gas volume flow patterns under different engineloading conditions. Thus, use of a fixed regenerator matrix geometryresults in variable flow losses and heat transfer characteristics,dependent on engine operating conditions. Because regenerators arenormally sized in order to satisfy some maximum operating condition,part-load efficiency may be improved by modifying the regenerator matrixrelative to the full-load requirements.

The difficulty of designing a regenerator system for a Stirling engineis further complicated by the fact that the time for a particle to passthrough the regenerator matrix is small compared to the total blow-time;in a Stirling engine, blow times are exceedingly short. For example, ata moderate engine speed of 1200 revolutions/min. or 20 cycles persecond, the blow time is 10 times less than the permissible minimum in agas turbine engine. Since the blow times are so short, it has beendemonstrated by other authors that no gas particle passes straightthrough the regenerator matrix in a single cycle. The actual net flowtime through the matrix is about half the complete cycle time, theremaining time being occupied in either filling or emptying, the deadspace. As a result, the heat transfer process that occurs is verycomplex, which involves a repetitive fluid to matrix, matrix to fluid,fluid to matrix cyclic relationship.

It is important therefore that the dead space and regenerator design beimproved to permit some adjustment to the changing flow pattern requiredunder different operating conditions.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide for variableregenerator and cooler capacity in a Stirling cycle engine.

Another object of this invention is to provide a Stirling cycleapparatus which is capable of varying the restriction to flow betweenthe expansion and compression spaces of the engine.

Features pursuant to the above objects comprise: (a) the use of morethan one set of a regenerator-cooler, and means for regulating flowthrough one or more of these sets; and (b) the employment of valves atthe entrance and exit of each of a plurality of such regenerator-coolerdevices, each of the valves being selectively controlled so that flowcan be passed through one or more of said arrangements.

SUMMARY OF THE DRAWINGS

The FIGURE is a schematic illustration of a portion of a working fluidsystem of a Stirling engine embodying the principles of this invention.

DETAILED DESCRIPTION

Turning to the FIGURE, a portion of a closed working fluid system 10 ofthe Stirling-type engine is shown having the pistons arranged in adouble acting manner. A plurality of cylinders, two of which are shownhere as 11 and 12, have the volume therein each respectively subdividedby pistons or reciprocating heads 13 and 14 so that each cylinder willhave the variable volume therein comprised of a high temperature (hot)space and a low temperature (cold) space. The hot space acts as anexpansion volume and the cold space acts as a compression volume. Forexample, with respect to cylinder 11, the hot space is identified as 15and the low temperature space as 16; with respect to cylinder 12, thehot space is identified as 17 and the low temperature space as 18. Eachhot space of one cylinder is connected by a suitable communicating means20 to a low temperature space of the next most adjacent cylinder. Suchcommunicating means comprises a gas passage 21 in which is interposed aplurality of regenerator-cooling apparatus, connected together by abifurcated passage 26 at their entrance and by a bifurcated passage 27at their exit. Each apparatus function in a known manner whereby gasdisplaced from the hot chamber passes through the regenerator (22 or 23)transferring heat units thereto and is thence cooled by coolingmechanism 24 or 25) before entering the low temperature space. Suchgases are again displaced during a subsequent phase of the Stirlingcycle, from the low temperature space back through the passage 21absorbing heat units from the regenerator, and again re-entering the hotchamber.

In a practical application, all gas particles may not undergo a completetranslation from the hot to the cold chambers, but rather there isthermal conduction that takes place through some of the gas medium thatis directed along such path.

The use of a pair of regenerator-cooler mechanisms connected in parallelas shown in FIG. 1, permits the use of two different Stirling cycleswithin the same engine. By placing valves 30 and 31 at the respectiveentrance and exit of one regenerator-cooler mechanism, the working gasflow through this mechanism can be shut off for low load or normal roadload engine operation. This would then allow the design of the engineusing only regenerator 23 and cooler 25 for mechanism for low load atmaximum efficiency. When higher loads are required, the valves can thenbe opened. The size or design of each regenerator-cooler mechanism ineach set could be different, particularly to achieve the condition ofmaximum efficiency or power; flow losses during the low load could bereduced to an optimum.

This invention is not an engine control method, but rather a mode ofobtaining better efficiency during most normal load conditions. Thevalves 30 and 31 can be either of the full closing type or of therestrictive type and may be solenoid operated by a remote control 28 orthe equivalent. Moreover, valves may alternatively be associated witheach of said regenerator-coolers so that a different design operation isachieved by restricting the flow to all of the regenerator-coolers.Thus, valves 30-31 as well as valves 30a-30a would be utilized incarrying out this alternative. Moreover, it may only be necessary toemploy one valve instead of the two as illustrated.

The control and operation of a double acting hot gas type Stirlingengine is more typically described in the prior art and specificreference herein is made to U.S. Pat. No. 3,859,792 which demonstrates asystem control whereby the main working pressure within said variablespaces is controlled to provide an increase or decrease of engine speedand torque.

I claim:
 1. A closed fluid working circuit for a regenerative typeStirling engine having a plurality of chambers subdivided bydouble-acting pistons operating therein, the subdivided chambers beingrespectively hot and cold and connected in series whereby a hot chamberis always in communication with a cold chamber of the next most adjacentcylinder, the improvement comprising:(a) means defining a plurality ofparallel arranged gas flow paths between the respective hot chamber andcold chamber of adjacent cylinders; (b) regenerator-cooler meansdisposed in each of said parallel arranged gas flow paths, and (c)control means for selectively permitting gas flow through one or more ofsaid regenerator-cooling means.
 2. The improvement as in claim 1, inwhich said control means is effective to restrict flow simultaneouslythrough each of said regenerator-cooler means.
 3. The improvement as inclaim 1, in which at least one of said regenerator-cooling means is notvalve controlled, but remains in the open fully communicated conditionat all times.